Method of making insulated resistors



May 12, 1942. s. B. EHRLICH METHOD OF MAKING INSULATED RESISTORS FiledOct. 24, 1940 INVENTOR.

WITNEJSE.

BY ffl; ATTO/mms.

Patented May 12, 1942 METHOD 0F MAKING INSULATED Y RESISTORS Sidney B.Ehrlich, St. Marys, Pa., signor to Stackpole Carbon Company, St. Marys,Pa., a

corporation oi.' Pennsylvania Application october 24, 1940, semi No.362,519

(ci. zal-s3) 3 Claims.

This invention relates to electrical resistance elements, and moreparticularly to molded iixed resistors that are insulated. y

Resistors of this character are made by molding a mixture' of powderedcarbon and a binder, such as Bakelite, into the form of smallcylindrical slugs the diameter and length of which depend upon thewattage dissipation desired. At the same time that the slug is molded,wire leads are embedded or molded in its ends so that the resistor canbe connected in an electric circuit. The slug is then baked to set thebinder and thereby form a strong and hard resistor body. The generalmanner of insulating these resistors has been to place them in forms andthen to mold resinous insulating coatings around them. 'Ihis yIlowingthrough it.

Another type of insulation that has been proposed is an insulating tubethat is slipped over the resistor body and then secured in place. It isdiillcult to obtain a tight lit with such a tube because it must beloose enough to slide over the resistor`body. It has also been proposedto wrap the resistor body with a narrow insulating tape wound spirallyaround the body, but this produces a long undesirable seam that may letin moisture,- and the completed element is not neat in appearance. Theends of the resistor are especially difficult to take'care of with sucha tape.

It is among the objects of this invention to provide an electricalresistor with inexpensive insulation which satisfactorily adheres to and-encloses the resistor body, which will. not admit of the tape it can beincreased by Wrapping thev tape two or more times upon itself. The tapeis held in place by an adhesive, and any tendency for the outer end ofthe tape to loosen can be avoided by coating the wrapped resistor with aliquid sealing material which is then hardened.

If the colored bands used in color coding the resistor are applied tothetape, the sealing nlm should be transparent so that they can be seenthrough it. The ends of the resistor can be in sulated by projecting thetape beyond the ends oi the resistor body in order to form cups that arethen filled with an insulating composition. To still more securely unitethe tape tothe resistor body pressure is applied to the tape at spacedpoints so that it has a corrugated appearance. In pressing the tape inthis manner it is stretched sideways and the portions adjacent its sidescaused to rmly adhere to the resistor body throughout its length.

'I'he invention is illustrated in the accompanying drawing in which Fig.l is a side view of one form of my invention; Fig. 2 is an enlarged endview thereof; Fig. 3 is a still further enlarged fragmentary sectionalview of the resistor: Fig. 4 is a side view, partly in section, of amodification of this invention; Fig. 5 is an end view of thismodiiication; and Fig. 6 is a fragmentary section through a thirdembodiment of the invention.

Referring to Figs. l, 2 and 3 of the drawing, a cylindrical resistorbody l, that has been molded from a mixture of powdered carbon and abinder. hasembedded in its ends theheads of pin-like wireleads 2 bywhich the resistor can be connected in an electric circuit. According tothis invention the resistor body is insulated by a tape 3 that is woundstraight around the body one or more timesdepending upon the amount oiinsulation desired. The tape is made of a suitable insulating materialthat has the necessary ilexibility and mechanical strength and that isnot aiiected by humidity conditions. The tape is caused to tightlyadhere to the resistor body by adhesive 4 previously applied to one ofthem, but preferably to the tape so that `successive layers of the tapewill adhere to one another.

The tape is of a width at least as great as the length of the resistorbody, and may be drawn from. a roll as it is wound around the resistor.After the tape has been wrapped around the resistor body, a plurality ofaxially spaced annular grooves are pressed into the tape. This pressesthe layers of tape still more tightly together and against the body andstretches them laterally beyond the ends of the body. Proper adhesionbetween the tape and the resistor body from end to end is thus assured.After the tape wrapping has been subjected to the heat dissipated by theresistor, it becomes hard and practically inseparable from the resistorbody.

The tape is protected, and its outer end further prevented from startingto unwind, by a coating film 5 of resinous material or the like which issprayed or painted on or otherwise applied to the wrapped resistor.Preferably, this material is air drying. The coating film is thicker inthe grooves in the tape than between them, and therefore forms aplurality of spaced reinforcing rings. When the resistor heats up in usethe tape yields sufficiently to avoid cracking of itself or the coatinglrn which also improves the electrical and humidity characteristics ofthe resistor.

In the modification shown in Figs. 4.and 5 the vtape 6 is wound onlyonce around the restistor body l to which it is secured by an adhesive1, although it can extend around it more times if desired. Also, thetape wrapping is uncorrugated and of a width substantially the same asthe length of the resistor body. To aid in preventing the outer end ofthe tape from peeling away from the resistor it is feathered, as shownin Fig. 5,

-and the entire wrapping is covered by a sealing film 8 similar tocoating 5 of the rst embodiment.

The color coding bands that indicate the resistance value of theresistor may be applied to the outer sealing lm in the usual manner. Or,they may be painted or printed on the tape before it is formed into aroll or as it leaves the roll, in which case the sealing film should betransparent, as shown in Fig. 4, such as methacrylic resin, so that thecode'bands 9 can be seen through the film. This method of applying thecolor bands is considerably cheaper than racking and then painting eachindividual resistor. It also eliminates the necessity of subjecting theresistor leads 2 to the undesirable atmosphere of a paint room in whichpaint and oxides resulting therefrom deposit on the leads and preventeasy soldering.

Fig. 6 -shows an embodiment of my invention in which, like in Fig. 3,the insulating tape Il extends beyond the end of the resistor body I andforms therewith a cup. This cup is filled with insulating material l2,-preferably a thermoplastic resin, whereby the ends of the resistor aresealed and insulated.

By using a tape which is wide enough to cover the resistor body from endto end, the tape can be wound straight around the body so that thewrapping is of substantially uniform thickness and a straight seam ofminimum length is provided. Also, the tape can be wound around the bodya plurality of times, if desired, which could not be done if it wereWound spirally as heretofore. The resistor body is thus most eiectivelyinsulated at a small cost, and its appearance is trim and neat.

According to the provisions of the patent statutes, I have explained theprinciple and construetion of my invention and have illustrated anddescribed what I now consider to represent its best embodiments.However, I desire to have it understood that, within the scope of theappended claims, the invention may be practiced otherwise than asspecifically illustrated and described.

I claim:

l. The method of making a resistor in which a molded body member isenclosed by an insulating tape member, comprising applying an adhesiveto one of said members, wrapping said tape member tightly andsubstantially straight around said body member, and then pressing aplurality of body member surrounding grooves in the tape member wherebyit is caused to rmly adhere to said body member.

2. The method of making a resistor in which a 4molded body member isenclosed by an insulating tape member, comprising applying an adhesiveto one of said members, Wrapping said tape member tightly andsubstantially 'straight around said body member, then pressing aplurality of body member surrounding groovesy in the tape member wherebyit is caused to firmly adhere to said body member, and coating the tapewith ya sealing material which lls said grooves.

i... edge portions caused to firmly adhere to said body.

SIDNEY B. EHRLICH.

